The invention relates to optoelectric distance measuring apparatus according to the principle of time of flight measurement, with a transmitter containing a laser diode, with a receiver comprising a photo diode for receiving the light impulses reflected from a target, with a pulse amplifier, with a pulse detector for the formation of the difference of the original pulse corresponding to the one reflected light impulse and a delayed copy thereof for the purpose of providing a stop signal for the point in time of reception of a reflected light impulse, and with a time circuit for communicating the time between the start of the emitted light impulse and the stop signal of the pulse detector.
A problem with short impulses is that they cannot be amplified in conformity with their true shape, so that the time-significant signal must be extracted alone from the leading edge, that remains true to the original even after amplification. A suitable method for this is described in the dissertation of A. Glasmacher's "Elektronische Schaltungen fur die Kernstrahlungsmesstechnik im Weltraum dagestellt am Zietmesskanal eines Massenspektrometers", Bochum 1978. According to this method with a so-called "constant fraction trigger", the delayed signal is multiplied by a factor-2, and the zero crossing of the sum of the original signal and the delayed signal is detected. This zero crossing of the sum appears when the leading edge has attained a constant fraction of the pulse amplitude and is thereby independent of the absolute magnitude of the amplitude.
In the circuit used, reflections take place both at the unloaded start and at the short-circuited end of a delay line. An impulse once introduced to the delay line will oscillate therein several times to-and-fro before being completely absorbed by the losses; as a result of this, premature light reflections, which for example originate from the lens or from back scatters in the air, can falsify the detection of the wanted signal.
In laser distance measuring apparatus according to the principle of time of flight measurement published in DE OS No. 26 34 627, a parallel resonance circuit as a working resistance is wired into the diode receiving circuit, the former being triggered across the receiving diode by the reflected light impulse, beginning to oscillate in accordance with its natural resonance. The stop signal is yielded from the intensity-independent zero crossings of the triggered sine signals.
With this distance measuring apparatus too, the detection of the desired impulse can be falsified by premature light reflections not emanating from the target, and also the receiver, compared with that of Glasmachers, is less noise-free and less sensitive.